Will increasingly frequent warm-water events in the Gulf of California reduce seabird populations? The case of Heerman’s Gull

Oceanic warm-water events in the Gulf of California have increased in frequency during the last three decades, passing from a historic mean of one or two warm anomalies per decade to five events in the 2007–2016 period. This can lead to massive failures in seabird nesting, as anomalously warm waters accumulate in the ocean’s surface, preventing the upwelling of colder, nutrient-rich waters from the ocean bottom, which in turn deprives seabirds of their food.

A recent study by Enriqueta Velarde from the University of Veracruz and Exequiel Ezcurra from the University of California Riverside, published in The Condor: Ornithological Applications, analyzed four decades of nesting dynamics of the threatened Heermann’s Gull (Larus heermanni), and modeled population growth under normal and high sea-surface temperature conditions.

Heermann’s Gulls exhibit the main characteristics of seabirds. They can live for decades, but take many years to reach breeding age, and produce few young. These traits allow their populations to resist oceanographic anomalies of up to one event every five years, but population size may decline rapidly if the frequency of warm-phase anomalies is over two per decade. Under normal conditions, predicted population growth is around 4%, with production of fledglings contributing to the population increase. Under anomalous warm-water conditions, population growth drops to -15% and adult survival becomes the key condition for the survival of the species. Simulations project a rapid population decline if warm anomalies maintain the high frequencies they have shown during the last decade.

These results underscore the need to understand the dynamics of warming ocean waters under current large-scale environmental change, as well as the importance of following the demographic dynamics of seabirds as indicators of oceanic conditions. Monitoring seabird populations may provide us with warning signals about the health and the future of our marine ecosystems.

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society, which merged with the American Ornithologists’ Union in 2016 to become the American Ornithological Society. In 2016, The Condor had the number one impact factor among 24 ornithology journals.